In contrast to the impressive body of information about serologically defined surface antigens of mouse leukemia cells, little is known about the non-H-2 antigens that cytotoxic t cells recognize on leukemia cells. In a study of leukemias induced by the radiation leukemia virus (RadLV) we have defined individually specific as well as crossreacting cell surface antigenic systems recognized by cytotoxic t cells. Questions asked in our current proposal are concerned with the relationship of these antigens to MuLV, the immunogenetics of their recognition, the heterogeneity of the T cell response, the relationship between T cell cytotoxicity in vitro and transplantation resistance in vivo, and the genetic origin of these antigens.